1. Field of the Invention
This invention relates to flywheels adapted for the storage of energy and more particularly to such flywheels having a laminar composite construction.
2. Description of the Prior Art
Recently, there has been a revival in the engineering and scientific interest in flywheels. This owes to the fact that flywheels provide an efficient means of energy storage with no adverse environmental impact. Flywheels may be useful as means for energy storage in solar energy systems, mechanical power systems, and electrical power systems. For example, electrical utilities may employ flywheels as a means of storing energy required for times of peak loading. Flywheels may also be useful as means for storing energy for propulsion and auxiliary power in air, land, sea and space vehicles.
Flywheels function by storing kinetic energy. The amount of energy capable of being stored in a particular flywheel is a function of the mass of the flywheel, the distribution of mass within the flywheel, and the maximum allowable speed of rotation of the flywheel. However, the maximum allowable speed of rotation of a flywheel is limited by the strength of the material from which the flywheel is formed. That is, as the rotational speed of a flywheel increases, the internal stresses within the flywheel also increase, which stresses, if allowed to exceed certain limits, would cause the flywheel to break apart or fracture. Therefore, optimally, flywheels should be constructed from materials having high strength to weight ratios.
Prior art flywheels, for the most part, have proven unsatisfactory in meeting energy storage requirements within certain constraints of mass and volume. For example, many prior art flywheels comprise discs or solid cylindrical members formed from a homogeneous metal and rotatable about the central axes thereof. Although these homogeneous metal flywheels are formed from high strength materials, the strength-to-weight ratios available in metal and the fracture mechanics of metal under cyclic fatigue conditions severely limit the flywheels' energy storage capabilities. Therefore, these prior art metal flywheels tend to be quite heavy. Moreover, should such prior art metal flywheel rupture, pieces breaking off the ruptured flywheel would possess sufficient energy to seriously damage equipment or injure persons in the vicinity. Such prior art metal flywheels are primarily useful where there are no size or weight constraints and where precautions have been taken to ensure the safety of persons or machinery in the area of the flywheel.
To overcome these disadvantages associated with prior art metallic flywheels, composite flywheels were developed. These composite flywheels are normally fabricated from a multiplicity of fibers of glass or similar materials disposed in a matrix or binder of epoxy resin or other suitable material.
Since such prior art composite flywheels are of a relatively low density as compared to prior art metallic flywheels, composite flywheels must be rotated at a higher velocity (i.e. tens of thousands of revolutions per minute) than a metallic flywheel of equal dimension to store an equivalent amount of kinetic energy. These prior art composite flywheels have for the most part been formed by a circumferential distribution of the fibers within the binder or matrix. Since, when a body rotates it is subject to stresses due to centrifugal force in a radially outward direction, such prior art composite flywheels must carry this centrifugal stress in directions normal to the axes of the fibers from which it is formed, placing the resin matrix in tension. Usually, epoxy and other resins employed with composite flywheels are relatively weak when loaded in tension as compared to the strengths of the fibers. Therefore, applying a high rotational speed to such a composite flywheel can cause the flywheel to break apart along circumferential lines between the fibers from which it is formed.
Prior art composite materials have also been formed from unified laminar constructions, each lamination comprising a multiplicity of parallel fibers embedded in a matrix of binder material and overlying an adjacent lamination such that the fibers of adjacent laminations are oriented at angles of either 60.degree. or 90.degree. with respect to each other. However, a flywheel formed from such prior art materials of these laminar constructions and particular angular orientations exhibits significant variations in load carrying abilities throughout its mass. For example, in a flywheel having a 60.degree. orientation between fibers of adjacent layers, if a first point is capable of carrying a particular value of centrifugal loading at a given radial strain (i.e. displacement), a second point at the same radius but displaced by 30.degree. from the first point will be able to accommodate only 87% of that loading at the same radial strain. Such a variation in strength and stiffness throughout a flywheel has the effect of introducing shear stresses and severely limiting the load carrying ability of the flywheel and therefore limits the energy storage capabilities of the flywheel.
Therefore, it is an object of the present invention to provide a cross-ply composite flywheel free of the deficiencies of prior art and capable of storing requisite amounts of energy within constraints of limited weight and volume.
It is another object of the present invention to provide a cross-ply composite flywheel wherein the risk of delamination or fracture at high rotational speeds is minimized.